[dartfuzz] Don't try to use FFI on simriscv.
[dartfuzz] Don't try to use FFI on simriscv.

TEST=dartfuzz
Bug: https://github.com/dart-lang/sdk/issues/49472
Change-Id: Ib3dc89b969197a8d27c330dfbcd4029dbf9dd22e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251848
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk